low watt guitar amp revisited

[kent]

Many moons ago you may recall that I was messing with the idea of a high gain low watt amp using those old car radio tubes with the 12V plates & heaters. Time is easing up a bit around the house so I'm slowly ressurecting the project. Now to the point...

Some earlier threads had mentioned using 70V transformers for ceiling mount paging speakers as output transformers. First of all, will this really work? Second, I'm having trouble finding any with a primary impedance listed. They're all spec'd at wattage. How can I find this out?

cheers,
kent

 

[Kev]

:green:
hi Kent

errr

don't worry about it
just get a simple roof 70V or 100V line trafo and try it

It can only not work !!
... and if it does work you have a cheap solution.

One of the manufacturers may have a web page with more spec's than just wattage .. as do the sellers and suppliers.

 

[NewYorkDave]

http://www.qsl.net/wa7zcz/area2/page11.html

 

[Scodiddly]

Wish I could remember the numbers, but I once did manage to figure out what sort of ratio lived in 70v transformers. I was building a little intercom amp, and needed to run a few 70 speakers. Those all came with transformers, but I needed one for the amp. I ended up using a multiple-speaker impedance matching transformer meant for running a few 8-ohm speakers off an 8-ohm amp. Something like 8:1? Anyway, I used that to make the amp output "70v" and it all worked fine. I think I found the info at MCM or Parts Express, something along the lines of how many watts at 8 ohms it took to get 70v at the transformer output.

 

[NewYorkDave]

This is not rocket science, folks. Doesn't anyone know Ohm's Law, damnit! :wink:

You know the voltage (70.7), you know the power... it's easy to figure out the impedance. See the link I posted above.

 

[PRR]

Ohm's Law tells you the nominal impedance.

What you also need to know is: most of these trannies have horribly low inductance, go short in the bass. Expect actual impedance to drop below 100Hz. But that's good enough for guitar. Especially since in a no-feedback pentode, speaker bass resonance is exagerated; shortening the transformer inductance is a classic way to tame the boom (and also save a dime).

Inductance will be less if you flow DC through the winding.

Also: a 70V winding is barely enough for a 150V plate supply. 70V RMS is 100V peak, plus 50V for plate-drop, 150V. But if you are low-volting the tubes to maybe 60V or less, a 25V winding may be more appropriate.

 

[NewYorkDave]

I had suggested a while ago that a 70V line transformer could work for a couple of low-power tubes in push-pull... but single-ended? Nah...

 

[kent]

Thanks for the feedback, guys.

I was thinking single ended. The 12k5 puts out a little more than a 1/4watt at 12V. Surely a 70V (or 25V) transformer could handle this.

http://hereford.ampr.org/cgi-bin/tube?tube=12K5

That's REALLY low watt but, hey, this is just an experiment, right?

 

[CJ]

Do you still have the link to the car radio tubes that guy was using for this amp?

 

[NewYorkDave]

It's not so much the voltage in this case, but the unbalanced plate current flowing through the secondary. In the "typical operation" example in this datasheet, it's 8mA
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/1/12K5.pdf

Since this is for a guitar amp, and we're more concerned with what "sounds good" than absolute fidelity... go for it! Try the 10W or 5W tap on the 70V transformer; that will get you close to the load impedance specified on the datasheet.

 

[CJ]

Surf's up!

http://www.jt30.com/jt30page/amplinks.html

 

[kent]

CJ-
http://sopht.250free.com/low-volume.html
I'm thinking of a Mesa Boogie mkI style front end. Granted - the 12u7 is closer to the 12au7 than an 12ax7 so I'll have to monkey around with the everything quit a bit.

Try the 10W or 5W tap on the 70V transformer; that will get you close to the load impedance specified on the datasheet.

I'm on it! Thanks Dave! I've had the tubes for months now so it'll be good to burn my fingers on an iron again!

cheers,
kent

 

[CJ]

Thanks!
That's it!
You just saved me from having to walk thru that link I posted!
Now off to that auto wreckers to get a 58 Mercury radio with all that chrome.
:guinness: :guinness: :guinness: :guinness: :guinness:
BTW, this was the last thread I was really into before being rudely interupted, if ya knows wats Isa sayin.
:twisted:

 

[CJ]

Sorry for the OT, don't want to start a new thread:
There is a UTC A-10 input on evilbay thats gone in 4 hours.
79 bucks buy it now, not bad.
I am surprised nobody has snatched it.
cj

 

[CJ]

OK howabout refurbing one of these into a practice amp?
The push buttons could be tone settings like I mentioned earlier.
These could be mounted in a wood cabinet.

 

[clintrubber]

That's an impressive looking gas-meter !
Deserves to be fully in chrome ?

 

[kent]

Hah! Too friggin' cool!